The development of a multimedia network has dramatically increased the demand for communications traffic. Therefore, for example, an optical transmission system that performs a multi-relay link by optically amplifying a wavelength division multiplexing (WDM) light by an optical amplifier using Erbium doped fiber (EDF) as an amplifying medium plays a large role in economization of communications system in the multimedia society.
Here, amplification efficiency of the EDF is large with respect to an input signal light in a C band (1530 nm to 1570 nm). Therefore, as a wavelength range of the WDM light, the C band is generally used. As a wavelength band of a pump light of the EDF, a band of 0.98 μm providing excellent noise figure (NF) and higher power in recent years may be used.
In an optical amplifier technology, for example, it is known that a structure in which a variable optical attenuator (VOA) is provided between a preceding-stage optical amplifying section (such as an EDF) and a following-stage optical amplifying section (such as an EDF). For example, Japanese Laid-open Patent Publication No. 2003-258346 discusses the following optical amplifier. The optical amplifier controls attenuation of a variable optical attenuator so that an output level of a preceding-stage gain controller that includes first and second optical amplifying sections and an output level of a following-stage gain controller that includes third and fourth optical amplifying sections become respectively a certain level. In addition, in order to distribute the attenuation so that the noise figure becomes an optimum value with respect to a plurality of variable optical attenuators when a device input level varies or when the amount of loss between the gain controllers varies due to dispersion compensation, the optical amplifier sets target gain values to respective second and fourth optical amplifying sections, to control the sum of gains from the first optical amplifying section to the fourth optical amplifying section to a certain value.
Japanese Laid-open Patent Publication No. 10-144984 discusses the following optical fiber amplifier. In the optical fiber amplifier, a signal light is amplified by using an Al added EDF to which pump light output from a pumping laser light source is supplied, the amplified signal light is then attenuated by a variable optical attenuator, and the signal light is further amplified by using a P/AI added EDF and the Al added EDF to which the pump light output from the pumping laser light source is supplied. Adding Al to the EDF makes it possible to equally disperse Erbium without crystallizing Erbium, and to widen an intense light generation band (1.55 μm band) in a C band. In addition, adding P to the EDF makes it possible to widen an amplification band in an L band.
In the above-described optical amplifiers, amplification gain in each EDF may be controlled to a certain value (auto gain control (AGC)) using a monitoring result obtained by monitoring the total power of the signal light (such as a WDM light) input to or output from each of the preceding-stage EDF and following-stage EDF. In addition, by changing the attenuation of the VOA provided between the EDFs, in accordance with variations in an input level of the signal light, the attenuation may be controlled so as to maintain a given output level while maintaining a flatness of an output wavelength characteristic with respect to the variations in the input level.
However, in the structure of each of the above-described optical amplifiers, a control loop of the AGC exists for each EDF. Therefore, the AGC may seem to be delayed by twofold when the number of waves of the input signal light is increased or decreased. In addition, in the structure of each of the above-described optical amplifiers, since the pump light source (such as a laser diode (LD)) is provided for each EDF, manufacturing costs of each optical amplifier is increased. Further, since pump light power demanded of each pump light source tends to increase, manufacturing costs of each pump light source may increase.